A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Stepping and/or scanning movements can be involved, to repeat the pattern at successive target portions across the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay (the accuracy of alignment between patterns formed in different patterning steps, for example between two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Methods and apparatus for determining structure parameters are, for example, disclosed in WO 2012126718. Methods and scatterometers are also disclosed in US20110027704A1, US2006033921A1 and US2010201963A1. In addition to scatterometry to determine parameters of a structure made in one patterning step, the methods and apparatus can be applied to perform diffraction-based overlay measurements. Diffraction-based overlay metrology using dark-field image detection of the diffraction orders enables overlay measurements on smaller targets. Examples of dark-field imaging metrology can be found in international patent applications US2010328655 A1 and US2011069292 A1. Further developments of the technique have been described in published patent applications US20110027704A, US20110043791A, US20120044470A US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. The above documents generally describe measurement of overlay though measurement of asymmetry of targets. Methods of measuring dose and focus of a lithographic apparatus using asymmetry measurements are disclosed in documents WO2014082938 A1 and US2014/0139814A1, respectively. The contents of all the mentioned applications are also incorporated herein by reference. The invention is not limited in application to any particular type of inspection apparatus, or even to inspection apparatuses generally.
A common problem in inspection apparatuses and other optical systems is one of controlling focusing of the optical system onto a target. Whether the optical system is for inspection by imaging, by scatterometry or for other purposes such as treatment of surfaces, many systems require real-time control of focus of the optical system, within very tight tolerances. A focus control arrangement for a scatterometer of the type described above is disclosed for example in published patent application US20080151228A. Light reflected from the target is imaged with deliberate focus error on two photodetectors. Comparing the illuminated area between the two photodetectors allows an indication of defocus to be obtained, and the direction of defocus to be identified. The contents of that application are incorporated herein by reference.
Current instruments using the known arrangement can achieve focus accuracy within around ±200 nanometers. However, the known arrangement also suffers from limitations in use. The focusing light to share the optical system with other radiations that relate to the main function of the optical system. These other radiations may be referred to as the working radiation to distinguish them from the focus control radiation. A single wavelength with limited power is generally used for focusing. However the working radiation being used by the instrument for exposure or inspection may be different, and focusing properties of the optical system at these different wavelengths may be different as a result. Known inspection apparatuses have mechanisms to apply an offset in the focus control arrangement, so that it can be used to focus the optical system for different wavelengths of working radiation.
One method of applying such an offset in the known focus control arrangement is to introduce an adjustable physical offset. This has the advantage of accurately shifting the focus by a known amount, but requires mechanical moving parts and causes delays when switching between different working radiation wavelengths. Accordingly, in some current apparatuses an electronic offset is introduced. This electronic offset can be switched instantaneously, but does not give an accurately known focus shift and reduces dynamic range of the focus control arrangement. There is therefore a desire for an improved electronic method of adjusting a focus control arrangement.
In a pending international patent application PCT/EP2015/070410, not published at the present priority date, a focus control arrangement with improved dynamic range and noise rejection can be obtained by applying an interferometric technique and lock-in detection in a focus control arrangement. Use of lock-in detection also allows different wavelengths of radiation to be used for focus monitoring, allowing good quality control over a wider range of targets. The techniques of the pending patent application can be employed in addition to the techniques disclosed below, if desired. The contents of the pending patent application are hereby incorporated by reference.